Add SingletonDerivativeModifier complement to TransformationSymmetry

[sethaxen]

IMP.core.TransformationSymmetry is an IMP.SingletonModifier that updates a list of IMP.core.XYZ, IMP.core.NonRigidMember and IMP.core.RigidBody coordinates/reference frames according to a list of reference particles.

A complementary IMP.SingletonDerivativeModifier with the proposed name TransformationSymmetryDerivatives should propagate the derivatives from the transformed particles back to the reference particles. This will enable gradient-based optimization such as molecular dynamics (#790).